The present invention relates to a transparent film suitable for formation of a toner image thereon by using a copying machine, a printer, etc., according to electrophotography, and a toner image forming method using the transparent film.
It has been generally practiced to form an image on a transparent film by using an electrophotographic copying machine and projecting the resultant image onto a screen by using an overhead projector (OHP). However, the image formation on a resinous film by electrophotography is accompanied with many problems compared with the image formation on paper. Accordingly, it has been practiced to form on the film surface a functional coating layer for solving the problems.
For example, the use of polyethylene terephthalate film as a typical transparent film for electrophotographic image formation thereon is accompanied with a known problem of conveyance failure, and for solving the problem, it has been practiced to provide an increased friction, e.g., by forming a surface coating layer containing a matting agent (Japanese Laid-Open Patent Application (JP-A) 1-315768, etc.).
Further, a polyethylene terephthalate film has a high surface resistivity, so that a high transfer bias voltage has to be applied, e.g., from a contact transfer member, in transferring a toner image on an image-bearing member, such as a photosensitive member, onto the film, and as a result thereof, defective images are liable to be formed due to abnormal discharge. In order to solve the problem, it has been practiced to suppress the surface resistivity to a certain level or below by surface-coating the film with an anti-static agent. This is also effective for suppressing conveyance failure due to charged attachment. Many proposals have been made regarding the surface resistivity adjustment. Among these, it is popular to apply anti-static agents onto film substrates. As anti-static agents, there have been proposed: ionically conducive agents, such as anionic antistatic agents, cationic antistatic agents and amphoteric antistatic agents; and electronically conductive agents, such as zinc oxide, tin oxide and titanium oxide (JP-A 62-94332, JP-A 6-75419, etc.).
As a result of image formation on such conventional films, the formation of defective images accompanied with lack of a portion of toner image, particularly in character images or thin-line images. This phenomenon is called xe2x80x9chollow image dropoutxe2x80x9d and is caused by a partial toner transfer failure during a transfer of toner image from a photosensitive member onto a transparent film, particularly at a thick toner portion at a middle part of a character pattern especially in case of outputting a high-resolution character image pattern, to result in a hollow image. It has been recognized that the xe2x80x9chollow image dropoutxe2x80x9d is particularly noticeably caused in an image forming machine adopting a contact transfer system using, e.g., a transfer roller.
Now, an example of electrophotographic image forming apparatus using a transfer roller is explained with reference to FIG. 5. Referring to FIG. 5, the image forming apparatus includes a photosensitive drum 1 as art image-bearing member, which is driven in rotation in an indicated arrow direction at a prescribed peripheral speed (process speed). During the rotation, the photosensitive drum 1 is uniformly surface-charged to a prescribed voltage of a prescribed polarity by a charger 2 (primary charging), and then exposed to image data-carrying light L supplied from imagewise exposure means (not shown), such as a laser beam scanning exposure means, or a focusing projection exposure image of an original image, to form an electrostatic latent image on its previously charged surface. The electrostatic latent image is developed by a developing device 4 to form a toner image Ta, which is transferred onto a transfer (-receiving) material P, such as a transparent film, supplied from a paper-supply means (not shown) at prescribed timing to a transfer nip n between the transfer drum 1 and a transfer roller 5 (as a contact transfer member) pressed against the drum 1, to provide a transferred toner image Tb on the transfer material P. Incidentally, the transfer material P is conveyed to and sent out from the transfer nip n along an entrance-side guide plate 6 and an exit-side guide plate 7 which are fixed to the apparatus main body.
Then, the transfer material P having passed the transfer nip n is separated from the photosensitive drum 1 surface and sent to a fixing device (not shown), where the toner image Tb is fixed onto the transfer material P to output an image product (print or copy). The surface of the photosensitive drum 1 after transfer is subject ed to removal of residual soiling substances, such as residual toner, by a cleaning device, and then recycled to a new image forming cycle starting from the primary charging. The transfer roller 5 comprises a core metal 5a and an electroconductive elastic layer 5b wound about the core metal 5a in the form of a roller (e.g., formed by molding). Longitudinal ends of the core metal 5a (shaft) are rotatably supported by a supporting member (not shown).
The transfer drum 5 is disposed in parallel with the photosensitive drum 1, pressed against the photosensitive drum 1 surface at a prescribed pressing force and rotated at an identical peripheral speed as the photosensitive drum 1. The transfer roller 5 is supplied with a bias voltage of a polarity opposite to that of the toner image Ta on the photosensitive drum 1 from a transfer bias voltage supply 9, whereby the transfer material P introduced to the transfer nip n is electrostatically charged, and the toner image Ta on the photosensitive drum 1 is transferred onto the transfer material P to provide the transferred toner image Tb thereon.
In the above-described transfer operation, the toner transferred and the transfer material P passing the transfer nip receive a prescribed pressure from the contact transfer member, whereby the toner particles are agglomerated. At this time, if a sufficient adhesion force operates between the toner and the transfer material P, the toner image Ta is satisfactorily transferred onto the transfer material P. However, if the adhesion force between the toner and the transfer material P is weak, and a rather stronger adhesion force acts between the toner and the photosensitive drum 1, the agglomerated toner can be brought to the drum 1 side. This difficulty is more liable to occur in the case where the transfer material P is a transparent film.
An object of the present invention is to provide a transparent film for image formation thereon by electrophotography capable of providing images free from image defects, such as hollow image dropout, lower image density and discharge pattern, which are liable to be caused by transfer failure or conveyance failure.
Another object of the present invention is to provide an image forming method using such a transparent film.
According to the present invention, there is provided a transparent film for electrophotographic image formation thereon, comprising a transparent substrate, and a surface-coating layer disposed on at least one surface of the transparent film; wherein said surface coating layer
exhibits a contact angle of 55-90 deg. with pure water at 23xc2x0 C. and a humidity of 50%RH,
contains an electronically conductive agent as a resistivity-adjusting agent, and
exhibits a surface resistivity of 1xc3x97108-1xc3x971012 ohm/xe2x96xa1.
According to another aspect of the present invention, there is provided-an electrophotographic toner image forming method, comprising transferring a toner image on an image-bearing member onto a transfer-receiving material, and fixing the toner image on the transfer-receiving material, wherein the transfer-receiving material comprises the above-mentioned transparent film according to the present invention.
In the transparent film according to the present invention, the surface coating layer is set to have a controlled contact angle in the range of 55-90 deg. and caused to contain an electronically conductive agent a a surface resistivity-adjusting agent, thereby solving the problem of hollow image dropout. Further, the surface coating layer is set to exhibit a surface resistivity of 1xc3x97108-1xc3x971012 ohm/xe2x96xa1, thereby providing an improved toner transfer efficiency and preventing the occurrence of defective images due to abnormal discharge, especially during the transfer of a toner image on the image-bearing member onto a transfer-receiving material while pressing the transfer-receiving material against the toner image on the image-bearing member by a contact transfer member.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.